A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment and its preparation method and application

Abstract

The present invention provides a magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment and its preparation method and application, and relates to the technical field of building materials. The raw materials of the grouting agent include: modified magnesium phosphate cement, water, citric acid, oxalic acid and borax, and the mass ratio of the modified magnesium phosphate cement, water, citric acid, oxalic acid and borax is (139.7-153.7): (25.1-27.7): (14.0-15.4): (14.0-15.4): (11.2-12.3). The present invention aims at the defects of the prior art, and solves the problem of slow initial setting of the grouting agent under low temperature conditions by utilizing the large amount of heat released by the hydration reaction of magnesium phosphate cement and good frost resistance. Since magnesium phosphate cement solidifies too quickly, resulting in poor fluidity, a silane coupling agent is added to solve the problem that the initial setting time of magnesium phosphate cement is too short, and the problem of poor fluidity is solved by neutralization of oxalic acid and alkali. It is suitable for the grouting needs of prestressed pipes under a low temperature environment of -20°C.

Description

Technical Field

[0001] The invention relates to the technical field of building materials, and in particular to a magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments, and a preparation method and application thereof. Background Art

[0002] In the current infrastructure construction, the proportion of highway and railway bridges is increasing. Among them, the construction of post-tensioned prestressed bridges is welcomed all over the world. Compared with other construction technologies, post-tensioned prestressed technology has more complicated processes, strong technicality, and certain difficulties in actual operation. The grouting process of prestressed ducts is a particularly important part of the prestressed construction process.

[0003] The corrugated pipe grouting material must have good density, which can effectively prevent steel corrosion and premature loss of prestress. At the same time, due to the low ambient temperature in the western plateau and cold regions, the strength of the hardened slurry will be too low, or even directly damaged by freezing, so the grouting material must also have good frost resistance.

[0004] When constructing in a low-temperature environment, the pipe grouting material must be able to condense and harden normally at low temperatures to meet the construction and use requirements of prestressed structures. In the prior art, conventional grouting materials are usually designed for positive temperature conditions. When used in an environment below zero degrees, problems such as prolonged condensation time, slow strength growth, or even inability to harden will occur. Although adding some antifreeze and accelerators can improve their performance to a certain extent, their performance in extremely low temperature environments (such as below -10°C) is often unstable and difficult to meet engineering requirements. At present, there are many studies on corrugated pipe grouting, but grouting materials for low negative temperature environments need to be further improved. In the current specifications, construction below 5°C is not allowed by the specifications. It is difficult for grouting materials to condense and harden normally in a negative temperature environment, and the volume will expand due to freezing, causing the prestressed pipe to crack, thereby posing a safety hazard.

[0005] Chinese patent application CN117800689A discloses a negative temperature pipeline grouting material, which is made by mixing cement, slag powder, microsilica powder, nano-silicon dioxide and other raw materials, and then adding water reducer, expansion agent, antifreeze agent, defoamer and auxiliary heat composition, using the auxiliary heat composition to quickly release heat and store heat through phase change, so as to ensure that the grouting material can be normally hydrated at low temperature and improve the early strength. However, it cannot be normally constructed at a low temperature of -20℃, otherwise the grouting material will not coagulate normally and the strength will be low.

[0006] Therefore, there is an urgent need for a prestressed pipe grouting material that has high setting strength, good fluidity, and appropriate initial setting time (meeting construction time requirements) in a negative temperature environment. Summary of the invention

[0007] The technical problem to be solved by the present invention is: the present invention overcomes the shortcomings of short setting time and poor fluidity of traditional magnesium phosphate cement materials, further improves the compressive and flexural strengths of magnesium phosphate cement, and at the same time, its preparation method is simple, low cost, and has good corrosion resistance, low temperature resistance, and volume stability, so that the modified magnesium phosphate cement can be suitable for working requirements in low temperature environments.

[0008] In order to solve the above problems, the specific technical solutions of the present invention are as follows:

[0009] A magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments, wherein the raw materials of the grouting agent include: modified magnesium phosphate cement, water, citric acid, oxalic acid and borax, wherein the mass ratio of the modified magnesium phosphate cement, water, citric acid, oxalic acid and borax is (139.7-153.7): (25.1-27.7): (14.0-15.4): (14.0-15.4): (11.2-12.3). If the water-cement ratio is too large, the compressive strength of the magnesium phosphate cement is too low under negative temperature conditions; if the water-cement ratio is too small, the setting time of the magnesium phosphate cement is slow and the fluidity is low. Borax is used as a retarder. If the borax content is too small, the magnesium phosphate cement has a poor retarding effect; if the borax content is too large, the compressive strength of the magnesium phosphate cement is low. Moderate amounts of citric acid and oxalic acid can improve the later development strength of the magnesium phosphate cement, while excessive amounts will reduce the later development strength.

[0010] Preferably, the modified magnesium phosphate cement is composed of magnesium oxide and modified diammonium phosphate, wherein the magnesium oxide is composed of dead-burned magnesium oxide and light-burned magnesium oxide, and the mass ratio of dead-burned magnesium oxide, light-burned magnesium oxide and modified diammonium phosphate is (100-110): (10-11): (29.7-32.7). The ratio of magnesium oxide to phosphate is positively proportional to the early strength of magnesium phosphate cement. The higher the ratio of magnesium oxide to phosphate, the better the mechanical properties. However, when it exceeds a certain range, the compressive strength will not increase all the time, but will increase first and then decrease. The ratio of magnesium oxide to phosphate is related to the setting time of magnesium phosphate cement. As the ratio of magnesium oxide to phosphate increases, the setting time of magnesium phosphate cement decreases. However, when it exceeds a certain range, the setting time will not decrease all the time.

[0011] Preferably, the modified diammonium phosphate is prepared by using diammonium phosphate as a raw material and modifying the surface of the diammonium phosphate by coating with a coupling agent.

[0012] Preferably, the coupling agent is an organic silicon compound coupling agent, and more preferably γ-aminopropyltriethoxysilane.

[0013] For magnesium phosphate cement, which is mainly composed of light-burned magnesium oxide (calcined at 900℃, activity 70%), heavy-burned magnesium oxide (calcined at 1600℃) and diammonium phosphate (industrial grade, purity 99%), the proportion of each of its main raw materials has the greatest impact on its mechanical properties. Struvite (MgNH4PO4·6H2O) generated by the reaction of various main raw materials is the main product that affects its strength. The reaction equation is as follows:

[0014] MgO + 2H + =Mg 2+ +H2O

[0015] MgO + NH4H2PO4 +5H2O= MgNH4PO4·6H2O

[0016] Therefore, the strength properties of the specimens were regulated by adjusting the phosphorus-magnesium ratio, the ratio of dead-burned magnesium oxide to light-burned magnesium oxide, and the water-cement ratio. After many tests, the mass ratio of magnesium oxide to modified ammonium dihydrogen phosphate was finally set at 4:1, the amount of light-burned magnesium oxide was 10% (mass) of dead-burned magnesium oxide, the amount of borax was 10% (mass) of dead-burned magnesium oxide, and the water-cement ratio was 0.18.

[0017] In the reaction mechanism of magnesium phosphate cement, ammonium dihydrogen phosphate (NH4H2PO4) has a high solubility and will ionize a large amount of H + and PO4 3- , causing the pH value of the solution to begin to decrease. Under this environment, the magnesium oxide powder will dissolve rapidly and produce a large amount of Mg 2+ and OH - , and the released Mg 2+ It will further react with H 4+ PO4 3- In order to extend the setting time of magnesium phosphate cement, it is necessary to prevent magnesium oxide from reacting with diammonium phosphate and water.

[0018] The reaction equation for the decomposition of diammonium phosphate is as follows:

[0019] NH4H2PO4 → H2PO4 - +NH4 +

[0020] H2PO4 - HPO4 2- +H +

[0021] HPO4 2- PO4 3- +H +

[0022] The reaction equation for the decomposition of magnesium oxide is as follows:

[0023] MgO+H2O → MgOH + +OH -

[0024] MgOH + +2H2O → Mg(OH)2+H3O +

[0025] Mg(OH)2 → Mg 2+ +2OH -

[0026] The reaction equation for the formation of struvite is as follows:

[0027] NH4H2PO4+MgO+5H2O → NH4MgPO4·6H2O

[0028] Therefore, the present invention adopts a method of coating the surface of diammonium phosphate with a coupling agent, which hinders the contact between diammonium phosphate and water molecules, and achieves a layer of surface coating to prevent the invasion of water molecules, thereby achieving the effect of slowing down the hydration rate of diammonium phosphate, and then prolonging the hydration reaction of magnesium oxide. The purpose of increasing the setting time of magnesium phosphate cement is achieved, so that it can meet the requirements of corrugated pipe grouting at -20°C.

[0029] Citric acid, oxalic acid and borax are used as retarder and early strength agent. As organic acids, their retarding effect on magnesium phosphate cement mainly occurs in the molecular structure based on the formation of COOH complex. 2+ It is a divalent cation with a coordination number of 4. It has the property of binding with weak acids and can form complexes in acidic environments. In the alkaline medium of the hydration product of magnesium phosphate cement, the carboxyl group and the free Mg 2+ Forming a protective coating, early hydration controls the Mg 2+ The dissolution rate of the drug increases, resulting in a prolonged coagulation time.

[0030] In addition, the present invention also provides a method for preparing the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environments, comprising the following steps:

[0031] S1. Preparation of modified ammonium dihydrogen phosphate:

[0032] Weigh ammonium dihydrogen phosphate and γ-aminopropyltriethoxysilane coupling agent according to a ratio, pour the coupling agent into a container containing ammonium dihydrogen phosphate and stir, and after stirring, dry the product to obtain modified ammonium dihydrogen phosphate.

[0033] S2. Preparation of grouting agent:

[0034] (1) The modified ammonium dihydrogen phosphate prepared in S1 is mixed with light-burned magnesium oxide and heavy-burned magnesium oxide to obtain modified magnesium phosphate cement.

[0035] (2) Add the modified magnesium phosphate cement prepared in step (1), water, citric acid, oxalic acid and borax into the reaction kettle, stir to obtain the grouting agent, put the slurry into a mold to form, and finally put it into a -20°C chamber for curing.

[0036] Preferably, in S1, the stirring time is 15-20 min, the drying temperature is 100-110° C., and the drying time is 2-4 h.

[0037] Preferably, in step (1) of S2, the mixing and stirring time is 3-5 min.

[0038] Preferably, in step (2) of S2, stirring is first performed at a speed of 120-150 r / min for 90 s, and then stirring is performed at a speed of 250-300 r / min for 120 s.

[0039] The present invention also provides the application of the magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments, and the application environment temperature is ≥-20°C.

[0040] The present invention has the following beneficial effects:

[0041] (1) In the present invention, the mass ratio of magnesium oxide to modified ammonium dihydrogen phosphate is set at 4:1, the amount of light-burned magnesium oxide is 10% of that of dead-burned magnesium oxide, the amount of borax is 10% of that of dead-burned magnesium oxide, and the water-cement ratio is 0.18, which can significantly improve the compressive strength and flexural strength of cement.

[0042] (2) The present invention utilizes magnesium phosphate cement, which releases a large amount of heat during hydration reaction, has good frost resistance, and has stable volume under different environments. Using it as a grouting agent in a low-temperature environment can effectively solve the problem of freeze-thaw damage.

[0043] (3) In the present invention, a coupling agent is added to the magnesium phosphate cement. The coupling agent coats the surface of magnesium oxide so that magnesium oxide does not undergo hydration reaction immediately. Compared with traditional magnesium phosphate cement, the modified magnesium phosphate cement has better fluidity and significantly prolonged setting time.

[0044] (4) The raw material cost of the grouting agent prepared by the present invention is low, and the raw material acquisition and preparation methods are simple, which can well meet the demand for grouting materials in low-temperature environments and is of great significance for the grouting of prestressed pipes in high-cold areas.

[0045] Therefore, the present invention aims at the defects of the prior art and solves the problem of slow initial setting of grouting agent under low temperature conditions by utilizing the large amount of heat released by the hydration reaction of magnesium phosphate cement and its good antifreeze property. Since magnesium phosphate cement solidifies too quickly, resulting in poor fluidity, silane coupling agent is added to solve the problem of too short initial setting time of magnesium phosphate cement, and oxalic acid is used to neutralize the poor fluidity. It is suitable for the grouting needs of prestressed pipes in a low temperature environment of -20°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0047] Figure 1 This is a graph showing the effect of curing temperature on the compressive strength of the grouting agent of the present invention.

[0048] Figure 2 This is a graph showing the effect of curing temperature on the flexural strength of the grouting agent of the present invention.

[0049] Figure 3(a) shows the change in compressive strength of the grouting agent of the present invention at different pre-curing ages, Figure 3(b) shows the change in compressive strength of the grouting agent of the present invention at different pre-curing ages, and Figure 3(c) shows the loss rate of flexural and compressive strength of the grouting agent of the present invention at different pre-curing ages.

[0050] FIG. 4( a ) is an XRD diagram of the grouting agent of the present invention after molding, and FIG. 4( b ) is the content of each phase of the hydration product of the grouting agent of the present invention.

[0051] Figure 5(a) is a SEM picture of the grouting agent of the present invention at a magnification of 100, Figure 5(b) is a SEM picture of the grouting agent of the present invention at a magnification of 1000, Figure 5(c) is a SEM picture of the grouting agent of the present invention at a magnification of 2000, and Figure 5(d) is a SEM picture of the grouting agent of the present invention at a magnification of 10000.

[0052] Figure 6 This is a flow chart of the grouting agent test of the present invention. DETAILED DESCRIPTION

[0053] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0054] Example 1

[0055] A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment comprises the following components by weight: 628.9g of modified magnesium phosphate cement, 111.4g of water, 61.9g of citric acid, 61.9g of oxalic acid and 49.5g of borax. The weight components of the modified magnesium phosphate cement are: 450g of dead-burned magnesium oxide, 45g of light-burned magnesium oxide, 124g of ammonium dihydrogen phosphate and 9.9g of coupling agent.

[0056] The preparation method of the grouting agent comprises the following steps:

[0057] S1. Preparation of modified ammonium dihydrogen phosphate:

[0058] (1) Weigh ammonium dihydrogen phosphate and γ-aminopropyltriethoxysilane coupling agent according to the proportion. After weighing, pour the coupling agent into the glass cup containing ammonium dihydrogen phosphate and stir rapidly for 15 to 20 minutes. After stirring, put it into a hot dryer and dry it at 105°C for 2.5 hours, then take it out and dry it to room temperature.

[0059] S2. Preparation of grouting agent:

[0060] (1) Add the air-dried modified ammonium dihydrogen phosphate to the light-burned magnesium oxide and the dead-burned magnesium oxide, and mix and stir for 3-5 minutes to obtain the prepared modified magnesium phosphate cement.

[0061] (2) Add modified magnesium phosphate cement, water, citric acid, oxalic acid and borax to the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into a 70.7 mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0062] Example 2

[0063] A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment comprises the following components by weight: 625.2g of modified magnesium phosphate cement, 111.4g of water, 61.9g of citric acid, 61.9g of oxalic acid and 49.5g of borax. The weight components of the modified magnesium phosphate cement are: 450g of dead-burned magnesium oxide, 45g of light-burned magnesium oxide, 124g of ammonium dihydrogen phosphate and 6.2g of coupling agent.

[0064] The preparation method of the grouting agent comprises the following steps:

[0065] S1. Preparation of modified ammonium dihydrogen phosphate:

[0066] (1) Weigh ammonium dihydrogen phosphate and γ-aminopropyltriethoxysilane coupling agent according to the proportion. After weighing, pour the coupling agent into the glass cup containing ammonium dihydrogen phosphate and stir rapidly for 15 to 20 minutes. After stirring, put it into a hot dryer and dry it at 105°C for 2.5 hours, then take it out and dry it to room temperature.

[0067] S2. Preparation of grouting agent:

[0068] (1) Add the air-dried modified ammonium dihydrogen phosphate to the light-burned magnesium oxide and the dead-burned magnesium oxide, and mix and stir for 3-5 minutes to obtain the prepared modified magnesium phosphate cement.

[0069] (2) Add modified magnesium phosphate cement, water, citric acid, oxalic acid and borax to the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into a 70.7 mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0070] Example 3

[0071] A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment comprises the following components by weight: 633.9g of modified magnesium phosphate cement, 111.4g of water, 61.9g of citric acid, 61.9g of oxalic acid and 49.5g of borax. The weight components of the modified magnesium phosphate cement are: 450g of dead-burned magnesium oxide, 45g of light-burned magnesium oxide, 124g of ammonium dihydrogen phosphate and 14.9g of coupling agent.

[0072] The preparation method of the grouting agent comprises the following steps:

[0073] Preparation of modified diammonium phosphate:

[0074] S1. Weigh ammonium dihydrogen phosphate and γ-aminopropyltriethoxysilane coupling agent according to the proportion. After weighing, pour the coupling agent into the glass cup containing ammonium dihydrogen phosphate and stir quickly for 15 to 20 minutes. After stirring, put it into a hot dryer and dry it at 105°C for 2.5 hours, take it out and dry it to room temperature.

[0075] S2. Preparation of grouting agent:

[0076] (1) Add the air-dried modified ammonium dihydrogen phosphate to the light-burned magnesium oxide and the dead-burned magnesium oxide, and mix and stir for 3-5 minutes to obtain the prepared modified magnesium phosphate cement.

[0077] (2) Add modified magnesium phosphate cement, water, citric acid, oxalic acid and borax to the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into a 70.7 mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0078] Comparative Example 1

[0079] A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment includes the following components by weight: 619g magnesium phosphate cement, 111.4g water, 61.9g citric acid, 61.9g oxalic acid and 49.5g borax. The weight components of magnesium phosphate cement are: 450g dead-burned magnesium oxide, 45g light-burned magnesium oxide and 124g diammonium phosphate.

[0080] Preparation of grouting agent: Add magnesium phosphate cement, water, citric acid, oxalic acid and borax into the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into 70.7mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0081] Comparative Example 2

[0082] A magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments comprises the following components in parts by weight: 450 g of heavy-burned magnesium oxide, 45 g of light-burned magnesium oxide, 124 g of diammonium phosphate, 111.4 g of water, and 49.5 g of borax.

[0083] Preparation of grouting agent: First, fully mix heavy-burned magnesium oxide, light-burned magnesium oxide and ammonium dihydrogen phosphate to prepare magnesium phosphate cement, then add magnesium phosphate cement, water and borax into the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into 70.7mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0084] Comparative Example 3

[0085] A magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments comprises the following components in parts by weight: 450 g of heavy-burned magnesium oxide, 45 g of light-burned magnesium oxide, 165 g of ammonium dihydrogen phosphate, 118.8 g of water, and 52.8 g of borax.

[0086] Preparation of grouting agent: First, fully mix heavy-burned magnesium oxide, light-burned magnesium oxide and ammonium dihydrogen phosphate to prepare magnesium phosphate cement, then add magnesium phosphate cement, water and borax into the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into 70.7mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0087] Comparative Example 4

[0088] A magnesium phosphate cement prestressed pipe grouting agent suitable for low-temperature environments comprises the following components in parts by weight: 495 g of dead-burned magnesium oxide, 165 g of ammonium dihydrogen phosphate, 118.8 g of water, and 52.8 g of borax.

[0089] Preparation of grouting agent: First, fully mix heavy-burned magnesium oxide, light-burned magnesium oxide and ammonium dihydrogen phosphate to prepare magnesium phosphate cement, then add magnesium phosphate cement, water and borax into the reactor, put it into the mixer and stir it at low speed for 90 seconds, then stir it at high speed for 120 seconds, and then put the prepared slurry into 70.7mm 70.7mm The product is formed by a 70.7mm mold and finally placed in a -20℃ chamber for curing.

[0090] Table 1 Grouting agent components

[0091]

[0092] Table 2 Comparison of grouting agent performance

[0093]

[0094] The grouting agents prepared in the embodiments of the present invention and the comparative examples are all cured at -20°C. According to the data comparison between Example 2 and Comparative Example 1 in Table 2, the initial setting time and fluidity of the modified magnesium phosphate cement are significantly improved with the addition of the coupling agent; it can be seen from Comparative Examples 2 and 3 that the flexural strength and compressive strength of the grouting agent prepared according to the phosphorus-magnesium ratio of the present invention are improved to a certain extent. Therefore, the grouting material of the present invention can be better applied to the grouting projects of bridges and prestressed pipes in high-latitude cold areas and plateau cold areas.

[0095] The present invention tests the strength performance of the modified magnesium phosphate cement grouting agent under different low temperature environments, and the test uses the grouting agent described in Example 1. Before preparing the grouting agent, the raw materials and test equipment are placed in a freezer at -10°C, -15°C, and -20°C. After being stored for 24 hours, the grouting agent is taken out and prepared in the manner described in Example 1. The prepared test blocks are then placed in a freezer at -10°C, -15°C, and -20°C for curing, and then taken out for mechanical property testing. The trends of mechanical properties under different temperature environments are shown in Figure 2. Figure 1 and Figure 2 shown.

[0096] Figure 1 This is the effect of curing temperature on the compressive strength of the grouting agent in Example 1. Figure 2This is the effect of curing temperature on the flexural strength of the grouting agent in Example 1. It can be seen that under different curing temperatures, the compressive strength and flexural strength of the grouting agent of the present invention do not change much. By comparing the strength performance under -10°C and -20°C environments, it can be seen that the strength performance of the specimens under -20°C environment decreased by about 12% in the early stage, and the strength performance decreased by about 10% in the later stage. The decrease is not large, which can well meet the demand for the strength of the grouting agent under low temperature environment.

[0097] In order to better meet the requirements of grouting materials in low temperature areas, the present invention also conducts freeze-thaw resistance test on the grouting agent prepared in Example 1. The size of the modified magnesium phosphate cement specimen for freeze-thaw test is 40 mm. 40mm The 160mm rectangular specimen was tested with the TYC-HDK concrete rapid freeze-thaw testing machine. Before the specimens were frozen and thawed, they were placed in a standard sample box (20°C) and cured for 12h, 1d, 3d, and 7d respectively. After the curing time was reached, they were placed in the freeze-thaw testing machine for freeze-thaw tests. A freeze-thaw cycle was set to 4 hours, and the freeze-thaw cycle time was 7 days.

[0098] After the 7-day freeze-thaw cycle test, the changes in the flexural and compressive strength and the loss rate of the grouting agent specimens of Example 1 at different pre-curing ages are shown in Figure 3(a)-Figure 3(c) As shown, Figure 3(a) shows the change in compressive strength, Figure 3(b) shows the change in flexural strength, and Figure 3(c) shows the loss rate of flexural and compressive strength.

[0099] from Figure 3(a) to Figure 3(c) It can be seen that the longer the pre-curing time of the modified magnesium phosphate cement is, the lower the loss rate of its flexural and compressive strength is, which means that the longer the pre-curing time is, the less free water is inside the specimen. After experiencing freeze-thaw cycles, the strength loss rate is in the range of 16.8% to 25.3%. This is because the hydration reaction of magnesium phosphate cement is mainly concentrated in the early stage. Most of the hydration reaction has been completed before the free water inside the specimen condenses into ice, and begins to condense to form strength. In the later stage, there is less free water inside the specimen, and the degree of damage caused by the freeze-thaw cycle to the specimen is also reduced.

[0100] Figures 4(a) and 4(b) are the XRD test results of the grouting agent of Example 1 after molding, where Figure 4(a) is the phase composition of the hydration product after the grouting agent is molded, indicating that the product after the modified dihydrogen ammonium phosphate reacts with water is consistent with the product before modification, indicating that the silane coupling agent KH-550 only covers the surface of dihydrogen ammonium phosphate and does not produce new substances with it in water. Figure 4(b) is the content of each phase of the hydration product after the grouting agent is molded. It is found that the content of NH4MgPO4·6H2O (struvite) in the modified magnesium phosphate cement specimen is 30.8%, while the content of struvite in the unmodified specimen is 35.6%. Therefore, the use of silane coupling agent KH-550 has little effect on the strength of magnesium phosphate cement.

[0101] The present invention conducted a SEM (scanning electron microscope) test on the grouting agent of Example 1 after molding. Figure 5(a) to Figure 5(d) As shown. As can be seen from Figure 5(a), after the grouting agent specimen is magnified 100 times, some holes appear on the surface; as can be seen from Figure 5(b), after magnification 1000 times, some struvite crystals are in block shape, and some struvite presents a columnar structure; as can be seen from Figures 5(c) and 5(d), the overall structure of the struvite is complete, the surface is smooth, there are no obvious cracks and holes, and the crystal particles are closely connected to each other, with obvious bridging effect to build the skeleton of the modified magnesium phosphate cement. Therefore, by analyzing and comparing its mechanical properties and microstructure, it can be obtained that even if the grouting agent specimen of the present invention is weaker than the unmodified magnesium phosphate cement specimen in terms of strength, the difference is very small.

[0102] Figure 6 The figure is a test flow chart of the grouting agent of the present invention. First, by curing magnesium phosphate cement, sulphoaluminate cement and silicate cement at -20°C for 3 days, by comparing their compressive and flexural strength, it is found that the performance of the test group using magnesium phosphate cement is far better than the other two groups, so magnesium phosphate cement is determined as the raw material of the grouting agent. In magnesium phosphate cement, the dosage of phosphate and magnesium oxide affects the performance of magnesium phosphate cement. The magnesium phosphate cement groups with different phosphorus-magnesium ratios are subjected to flexural strength test, compressive strength test, fluidity test and setting time test to obtain the optimal phosphorus-magnesium ratio. Since LBM (lightly burned magnesium oxide) has higher activity and faster reaction rate than M (heavy burned magnesium oxide), three groups of magnesium phosphate cements with different LBM / M are subjected to flexural strength test, compressive strength test, fluidity test and setting time test to obtain the optimal LBM / M. The optimal dosage of coupling agent is obtained by the magnesium phosphate cement with different dosages of coupling agent in the flexural strength test, compressive strength test, fluidity test and setting time test. Therefore, the best mix ratio of modified magnesium phosphate cement basic raw materials is obtained.Finally, the mechanical properties of modified magnesium phosphate cement are tested by -10~-20℃ negative temperature grouping performance test and freeze-thaw resistance test.

[0103] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environments, characterized in that: The raw materials of the grouting agent include: modified magnesium phosphate cement, water, citric acid, oxalic acid and borax, and the mass ratio of the modified magnesium phosphate cement, water, citric acid, oxalic acid and borax is (139.7-153.7): (25.1-27.7): (14.0-15.4): (14.0-15.4): (11.2-12.3).

2. The magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environments according to claim 1, characterized in that: The modified magnesium phosphate cement is composed of magnesium oxide and modified diammonium phosphate, wherein the magnesium oxide is composed of dead-burned magnesium oxide and light-burned magnesium oxide, and the mass ratio of the dead-burned magnesium oxide, the light-burned magnesium oxide and the modified diammonium phosphate is (100-110): (10-11): (29.7-32.7).

3. The magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 2, characterized in that: The modified diammonium phosphate is prepared by using diammonium phosphate as a raw material and coating and modifying the surface of the diammonium phosphate with a coupling agent.

4. The magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 3, characterized in that: The coupling agent is an organic silicon compound coupling agent.

5. The magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 4, characterized in that: The coupling agent is γ-aminopropyltriethoxysilane.

6. The method for preparing the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Preparation of modified ammonium dihydrogen phosphate: Weighing ammonium dihydrogen phosphate and γ-aminopropyltriethoxysilane coupling agent, pouring the coupling agent into a container containing ammonium dihydrogen phosphate and stirring, and drying the product after stirring to obtain modified ammonium dihydrogen phosphate; S2. Preparation of grouting agent: (1) mixing the modified ammonium dihydrogen phosphate prepared in S1 with light-burned magnesium oxide and heavy-burned magnesium oxide to obtain modified magnesium phosphate cement; (2) Add the modified magnesium phosphate cement prepared in step (1), water, citric acid, oxalic acid and borax into the reaction kettle, stir to obtain the grouting agent, put the slurry into a mold to form, and finally put it into a -20°C chamber for curing.

7. The method for preparing the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 6, characterized in that: In S1, the stirring time is 15-20 minutes, the drying temperature is 100-110°C, and the drying time is 2-4 hours.

8. The method for preparing the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 6, characterized in that: In step (1) of S2, the mixing and stirring time is 3-5 minutes.

9. The method for preparing the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environment according to claim 6, characterized in that: In step (2) of S2, stirring is first performed at a speed of 120-150 r / min for 90 s, and then stirring is performed at a speed of 250-300 r / min for 120 s.

10. Use of the magnesium phosphate cement prestressed pipe grouting agent suitable for low temperature environments according to any one of claims 1 to 5, wherein the application environment temperature is ≥-20°C.

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